Alloy



Patente ,5

Louis W..Kempf, Lakewood, and Ivan B. Dawson, Cleveland, Ohio, gnors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Peylvania No Drawing. Application June 2, 1982 Serial No. 015,0t8

The invention relates to metallic alloys containing substantial amounts of aluminum andsilicon and smaller amounts, if desirable, of other elements and compressed from comminuted metallic constituents.

In the art of casting aluminum-silicon alloys containing substantial amounts of silicon the difiiculty encountered in making sound and serviceable castings increases with each addition of the alloying element silicon. As the silicon content of the alloy exceeds about 20 per cent, it becomes increasingly difiicult and economically impractical to make acceptable castings and, therefore, although aluminum alloys containing more than 20 per cent of silicon have very attractive properties, they have not heretofore been used to any extent because of the difiiculties encountered in casting.

An object of this invention is the provision of an alloy containing substantial amounts of aluminum and silicon and having a lower coefficient of thermal expansion than has heretofore been commercially obtainable in cast aluminum alloys. Another object of the invention is the provision of an alloy of a lower specific gravity than has heretofore been obtained by casting aluminumsilicon alloys. A further object of the invention is the provision of an alloy of exceptionally low specific gravity and coemcient of thermal expansion with other physical characteristics suited to uses where low thermal expansivity and light weight are particularly desirable. Another and important object of the invention is the provision or an alloy which has a high compression strength at elevated temperature.

We have discovered that very valuable alloys can be produced within the range of 20 per cent to about 90 per cent of silicon and about 10 per cent to about 80 per cent of aluminum by comminuting the alloying constituents and then, after thoroughly mixing the powders, compressing them into desired form under the influence of high pressure, with or without the use of elevated temperatures during or subsequent to compression, As an alternative, the comminuted mixture may be compressed into a suitable blank which approximates the final contour of the cast article, and the finished article may be machined therefrom.

From these compressed and homogenized products we have been enabled to obtain a lower specific gravity than has heretofore been commercially available in aluminum-silicon alloys, a lower coefiicient of thermal expansion than has heretofore been obtainable in any light alloy in (C1. MIL-21.1)

the aluminum base or magnesium base field, and a very high compression strength at elevated temperatures. By the addition of other metallic constituents, such as magnesium and beryllium,

to the compressed mixture, it is possible to fur- 00 ther decrease the specific gravity without materially impairing the low thermal expansivity or the high compressive strength of the original mixture. Likewise, by the addition of'other metallic constituents such as iron, manganese and the like, it is possible to alter or modify the properties of the compressed aluminum-silicon mixture to adapt the properties of the compressed product to a variety of uses.

In practicing our invention, a calculated amount of metallic powder, in a proportion to effect the desired final composition, is thoroughly mixed by mechanical .or manual means. More intimate alloying action is possible as the powder increases in fineness. The powder should be finer than about 40 mesh, as measured on the U. 8; Standard screen, and to produce a product of maximum desirable properties we prefer to use powders which are 90 mesh or finer. A good clean grade of powder as free as possible'from dirt or other impurities is especially desirable. A die of suitable shape is secured, the powder mixture placed in the die and pressure is brought to bear on the mixture to compress it into a solid homogeneous briquette or blank ready, if necessary, for a final machining operation. The dies in which the mixtures of aluminum powder and silicon powder are placed are preferably heated in order to reduce the amount of pressure necessary to the production of a sound, dense, homogeneous product. Three factors cooperate to improve the product; namely, temperature, pressure, and time at pressure. Raising the temperature, increasing the pressure or lengthening the time at pressure serves to produce a sounder,

cause we have determined that elevated tem- 1 0 peratures serve to produce a better product. Although the temperatures employed are not critical, we prefer to opeiate within a temperature range of about 200 centigrade to just below the temperature at which the lowest melting constituent," formed on alloying the comminuted mixture, will melt or undergo incipient fusion. Within these temperatures we prefer to use pressures in the order of about. 25,000 to 150,000

pounds per square inch, which pressures are applied to the die-confined metallic powder mixture for periods of time which will vary inversely with the temperature and pressures used. For instance, when the dies are heated to about 400 centigrade, a pressure of 100,000 pounds per square inch need not be maintained more than about 2 minutes in order to produce a sound, dense, homogeneous product. On the other hand, at temperatures of 200 centigrade, a pressure of 100,000 pounds per square inch should be applied for about 2 hours. The products obtained by subjecting comminuted mixtures of aluminum and silicon to high pressures at room temperature are commercially useful although they lack, from a standpoint of soundness, thev properties of similar compressed mixtures com pressed at elevated temperatures.

After the aluminum-silicon compressed mixture is removed from the die it may be homogenized and its physical properties furtherim-,

proved by a subsequent thermal treatment at an elevated temperature. This temperature should preferably be close to or immediately below the melting point of the lowest melting constituent of the compressed mixture, but temperatures as low as 300 centigrade give excellent commercial results.

Using a mixture containing 40 per cent of comminuted silicon and 60 per cent of comminuted aluminum and compressing this mixture under a pressure of 40,000 pounds per square inch at a temperature oi about 430 centigrade, we have made articles having a coeflicient of expansion oi 0.000015 perdegree centigrade. The compression strength of the alloy alter being heated for about 24 hours at 565 centigrade is about 48,860 pounds per square inch. The compression strength 01' the alloy immediately ai'ter compressing and in the unheat-treated condition is about 35,280 pounds per square inch.

' The specific gravity of the alloy is about 2.5 and about 400 centigrade had a compressive strength of 31,300 pounds per square'inch and a yield point of 28,400 pounds per square inch, the yield point being calculated from the stress sufficient to cause a 5 per cent reduction in.

length under compression. Alter the alloy had beenv heat-treated for 24 hours at 555 centigralde,

the yield point had increased to 33,000 pounds per square inch and the compressive strength to 47,000 pounds per square inch. This ailoyis particularly useful in that it has a coeflicient of thermal expansion about equal to that oi. cast iron or steel,-the coeflicient being 0.000010 per degree centigrade from room. temperature to 100 centigrade.

The novel products which we provide in accordance with our invention have certain unusual properties. 7 Since the granules of the various powdered elements are mixed thoroughly and compressed into a coherent, solid mass, no trouble is encountered with elements which might be immiscible or insoluble in the molten or solid condition after attempts were made to make a casting of the specific composition. Thus lesser amounts of alloying constituents may be added to the aluminum-silicon mixture to create therein specific properties heretofore not obtainable in cast aluminum-silicon alloys. The compressed alloys are likewise characterized by a high compressive strength at elevated tempera tures and there is thus provided a material of low specific gravity, low thermal expansivlty and high compressive strength; a combination of properties greatly desired where the material is formed into a part which must operate at elevated temperatures under compressive stresses.

In this specification and the appended claims the term alloy is used in the sense of a metallic material, not a chemical element, which may be composed entirely of two or more metallic elements or or a metallic element in combination with one or more other elements which may be metals or non-metals.

We claim:

1. An alloy consisting substantially of aluminum and-silicon compressed 'i'rom comminuted materials characterized by high compression strength and low thermal expansivity and containing from about 20 to about 90 per cent of silicon and from about 10 to about per cent of aluminum.

2. The method of forming an alloy consisting substantially of aluminum and silicon containing from about 10 to about 80 per cent 01' aluminum and from about 20 to about per cent 01 silicon by comminuting and thoroughly mixing the alloying constituents, subjecting the comminuted mixture to pressure to form the same into a dense, homogeneous mass, and heat-treating the alloy will fuse.

' LOUIS W. IVAN R. DAWSON. 

